1. Field of the Invention
The present invention relates to a method and an apparatus for inspecting an optical device and, more particularly, it relates to a method and an apparatus for inspecting an optical device, which are suitable for inspecting a wavefront of ultraviolet rays, X-rays, etc., passing through a predetermined optical system.
2. Description of the Related Art
For exposure apparatuses so far employed in a lithography process for producing semiconductor elements and so on, there have been increasing demands to effect improvements in imaging features of an optical projection system for transcribing an image of a mask pattern onto a board such as a wafer, in order to enable finer and finer detail to be produced. In order to achieve such improvements, it is required to measure a status of wavefront aberration of an optical projection system with a high degree of precision so that a Twyman-Green interferometer and a Fizeau interferometer have hitherto been employed for inspecting a wavefront of laser beams, each of which uses Hexe2x80x94Ne laser beams as an inspecting light as opposed to an exposing light employed conventionally. These measuring methods comprise measuring the status of a wavefront of light flux passed through an optical system to be inspected by causing the light flux passed through the optical device to be inspected to interfere with light flux from another reference plane by taking advantage of a very high interference performance inherent in Hexe2x80x94Ne laser beams.
However, a measuring method using two light fluxeses with different light paths suffers from the disadvantage that interference fringes are likely to undergo an irregular distortion due to influences of vibration, large-scale optical parts of high precision are required to form a reference plane of high precision, and it is difficult to adjust the two light paths. Therefore, a point diffraction interferometer (PDI) is proposed (e.g. as disclosed in Japanese Patent Application Laid-open (Kokai) No. 57-64,139) which is so adapted as to create a reference light from a portion of the light flux passed through the optical projection system to be inspected. The prior art point diffraction interferometer can use even an exposing light intact as an inspecting light, which is not so high in interference performance, and can adjust a light path with ease, because the light path of a measuring light is almost equal to the light path of a reference light.
Particularly, as an exposing light (exposing light energy beams), there have recently been employed far ultraviolet rays such as ArF excimer laser beams (having wavelength of 193 nm), too, in addition to ultraviolet rays such as KrF excimer laser beams (having wavelength of 248 nm). Further, reviews have been made to use F2 laser beams (having wavelength of 157 nm), soft X-rays (having wavelength of several tens nm to about 1 nm) or hard X-rays (having wavelength of below about 1 nm) as an exposing light. As the kind of glass materials having high transmission which can be applied to an exposing light with such short wavelengths are limited, the use of an optical projection system has been reviewed which combines e.g. a concave mirror or a plane mirror covered with a reflecting coating composed of a predetermined number of layers. Although quartz, fluorite and the like are known to be employed as glass materials for transmission of e.g. ArF excimer laser beams, it is required to improve performance of a transmission coating in order to enhance transmission of those materials when they are to be used as such glass materials. In order to inspect a wavefront of the optical projection system including characteristics of a reflecting coating or transmission coating, it is required to use an exposing light as an inspecting light. To this end, a point diffraction interferometer is suitable.
Such a point diffraction interferometer can present the advantages that an exposing light can be employed intact as an inspecting light and a light path can be readily adjusted upon effecting inspection of the wavefront of the optical projection system of the exposure apparatuses as stated above. Using this conventional point diffraction interferometer, however, it is difficult to measure a phase difference of interference fringes between certain two measurement points with high precision because such interference fringes yielded therewith remain static. Therefore, the conventional point diffraction interferometer poses the problems that it is difficult to further improve precision of inspecting the wavefront of the optical projection system and it is rather poor in the ability of reproducing results of inspection. In other words, as the conventional point diffraction interferometer can be provided with various advantages, it nevertheless is not sufficiently precise.
Likewise, among interferometers other than the point diffraction interferometer, an interferometer of the type (e.g. a zone plate interferometer) capable of producing a reference light from a portion of light flux passed through an optical system by taking advantage of an illuminating light (e.g., an exposing light, etc.) with which the optical projection system to be inspected is actually irradiated suffers from the defect that a SN ratio of interference fringes may be likely to become worse. Therefore, demands have been made to develop a technique for inspecting a phase of interference fringes at desired measurement points with high precision.
In light of the above, the present invention has a primary object to provide an inspection method and an inspection apparatus (an interferometer) for inspecting an optical device, which are so adapted as to detect a phase of interference light between light flux passed through the optical device as an object to be inspected and reference light to be produced from a portion of the light flux with high precision.
Further, the present invention has a second object to provide an inspection method and an inspection apparatus (an interferometer) for inspecting an optical device, which are adapted so as to detect a two-dimensional distribution of phases of interference light with high precision, even when there is employed a light flux having a lower interference performance as an illuminating light for detection.
The inspection method for inspecting the optical device in accordance with the present invention is directed to an inspection method for effecting inspection of a wavefront of light flux passed through the optical device, which comprises the step of passing the light flux through the optical device as an object to be inspected, and the step of producing a diffracting light from a portion of the light flux passed through the optical device through a diffracting element in the process of movement. The inspection method further comprises the step for detecting an interference light between the diffracting light and another light flux passed through the optical device during the movement of the diffracting element.
The diffracting element is a diffracting member having a pinhole shape, and it is preferred to generate the diffracting light with the pinhole-shaped diffracting member. The movement of the pinhole-shaped diffracting member may include vibration of the diffracting member in a direction intersecting the light flux passed through the optical device or vibration of the diffraction member in a direction along or parallel to the light flux passed through the optical device.
The inspection method for the optical device in accordance with the present invention comprises varying phases of the diffracting light generated by diffraction with the diffraction member by effecting the movement of the diffraction member. The diffracting light is a reference light having an ideal wavefront (an undistorted spherical plane). The movement of the diffraction member can periodically vary a phase difference between the diffracting light as a reference light and another light flux (having a wavefront distorted by influences of aberration with the optical device) passed through the optical device as an measuring light which is not diffracted by the diffracting light. The periodical variation in the phase difference between the diffracting light and the measuring light cannot only identify the direction of increasing or decreasing the degree of interference fringes (irregularity on a wavefront to be inspected) which could not be identified from static interfernce fringes obtained with the conventional point diffraction interferometers due to superimposition of fringes, but also interpolate a gap between the interference fringes to be created by interference between the diffracting light and the measuring light. Further, by comparing the phases of the interference light obtained at two locations, the phase difference in the interference light between the two locations can be detected with high resolution and precision. The interference light in this case is a heterodyne interference light.
The diffracting element may comprise a light path division device and a pinhole-shaped diffracting member, the light path division device being so adapted as to divide the light flux passed through the optical device into two light fluxes having different light paths and the pinhole-shaped diffraction member being disposed on the light path of one of the two light fluxes divided with the light path division device.
In this case, the light path division device is operated to move and the diffracting light is generated with the pinhole-shaped diffraction member during the movement of the light path division device. Further, during the movement of the light path division device, an interference light between the diffracting light generated with the pinhole-shaped diffraction member and the other light flux divided with the light path division device is detected. The light path division device may comprise, for example, a diffraction grating. The movement of the light path division device may comprise a continuous movement of the light path division device in a direction intersecting the light flux passed through the optical device.
When the diffraction grating as the light path division device is transferred continuously, the wavefront of the two light fluxes (diffracting light) divided with the diffraction grating are also transferred to effect frequency modulation, thereby changing a velocity of varying the phase of the wavefront of each light flux by the degree of the diffracting light, or periodically changing the phase difference of the two light flux generated from the diffraction grating. In this case, too, the interference light of the two light fluxes generated from the diffraction grating comprises a heterodyne interference light.
Moreover, the diffracting element may comprise a diffraction grating and a pinhole-shaped diffraction member, the diffraction grating being so adapted as to generate two light fluxes having different degrees of diffraction by diffraction of the light flux passed through the optical device and the pinhole-shaped diffraction member being disposed on the light path of one of the light fluxes generated from the diffraction grating.
In this case, the diffraction grating is operated to move and the diffracting light is generated with the pinhole-shaped diffraction member during the movement of the diffraction grating. During the movement of the diffraction grating, an interference light generated between the diffracting light generated with the pinhole-shaped diffraction member and the other light flux generated with the diffraction grating is detected. The movement of the diffraction grating may comprise a movement for stepwise transferring the diffraction grating by a predetermined pitch in the direction intersecting the light flux passed through the optical device.
Upon stepwise transferring the diffraction grating by a predetermined pitch, the phases of the two diffracting light are caused to change, thereby varying a phase difference of the resulting interference light periodically.
The optical device as an object to be inspected may comprise, for example, an optical projection device for projecting an image of a pattern formed on a first plane onto a second plane. The optical projection device may comprise an optical projection system disposed between a board and a mask in an exposure device for transcribing a pattern of the mask onto the board. The light flux incident into the optical device may comprise an exposing light to be irradiated upon the board through the mask and the optical projection system, and the exposing light may include ultraviolet rays or X-rays. Further, the optical projection system may comprise a reduced optical system having plural optical reflection elements.
Moreover, an alternative inspection method for inspecting an optical device in accordance with the present invention may include an inspection method for effecting inspection of a wavefront of light flux passed through the optical device, which may comprise a step for passing the light flux through the optical device as the object to be inspected, a step for changing a phase difference between a diffracting light obtained by diffracting a portion of the light flux passed through the optical device and the other light flux passed through the optical device, and a step for detecting an interference light between the diffracting light and the other light flux during the change of the phase difference between the diffracting light and the other light flux.
The portion of the light flux may be diffracted with the pinhole-shaped diffraction member. The phase difference between the diffracting light and the other light flux can be periodically changed by the movement of the pinhole-shaped diffraction member. The movement of the pinhole-shaped diffraction member may comprise vibration in the direction intersecting the light flux passed through the optical device or vibration in the direction along and in parallel to the light flux passed through the optical device.
On the other hand, the inspection apparatus for inspecting the optical device in accordance with the present invention may comprise a diffracting element for generating a diffracting light from a portion of the light flux, which is disposed on the light path of light flux passed through an optical device as an object to be inspected, a drive unit for driving the diffracting element, and a detection unit for receiving an interference light between the diffracting light and the other light flux passed through the optical device during movement of the diffracting element.
The diffracting element may comprise a pinhole-shaped diffracting member that generates the diffracting light. The drive unit may be configured so as to vibrate the pinhole-shaped diffraction member in the direction intersecting the light flux passed through the optical device or in the direction along and in parallel to the light flux passed through the optical device.
Further, the diffracting element may comprise a light path division device for dividing the light flux passed through the optical device into two light fluxes having different light paths, and the pinhole-shaped diffracting member for generating the diffracting light which is disposed on the light path of one of the two light flux divided with the light path division device. In this case, the light path division device may comprise a diffraction grating and the drive unit may be configured to continually transfer the light path division device in the direction intersecting the light flux passing through the optical device.
Moreover, the diffracting element may comprise a diffraction grating for generating two light fluxes having different diffraction degrees by diffracting the light flux passed through the optical device and a pinhole-shaped diffracting member for generating the diffracting light, which is disposed on the light path of one of the two light fluxes generated from the diffraction grating. In this case, the drive unit may be configured so as to stepwise transfer the diffraction grating by a predetermined pitch in the direction intersecting the light flux passed through the optical device.
Furthermore, an alternative inspection apparatus for inspecting the optical device in accordance with the present invention may comprise a diffracting element for generating a diffracting light from a portion of the light flux passed through the optical device as an object to be inspected, which is disposed on the light path of the light flux passed through the optical device, a unit for varying a phase difference between the diffracting light and the other light flux passed through the optical device, and a detection unit for detecting an interference light between the diffracting light and the other light flux during a variation in the phase difference with the unit. The diffraction member may comprise a pinhole-shaped diffraction member. The unit for varying the phase differences periodically may comprise, for example, a drive unit for vibrating the pinhole-shaped diffraction member in the direction intersecting the light flux passed through the optical device or in the direction along or in parallel to the light flux passed through the optical device.
Moreover, an alternative inspection apparatus for inspecting the optical device in accordance with the present invention may comprise a diffraction member in a pinhole form disposed on a light path of light flux (e.g., ultraviolet rays, X-rays, etc.) passed through or transmitted through or reflected from an object to be inspected, a drive system for vibrating the diffracting member in the pinhole form, and a detection system for receiving a heterodyne interference light between the light flux passed through the object to be inspected and the diffracting light generated with the diffracting member upon vibrating the diffracting member via the drive unit. The inspection apparatus is so configured as to detect optical characteristics of the object to be inspected on the basis of detection signals of the detection system (as illustrated in FIGS. 1 and 2).
The inspection apparatus according to the present invention can detect an interference light between the light flux passed through the object to be inspected as a measuring light and the diffracting light generated from a portion of the light flux with the diffracting member as a reference light. Upon this instance, a heterodyne interference light can be obtained by changing a phase of the reference light by vibrating the diffracting member in the direction (D1) intersecting the running direction of the light flux from the object to be inspected or in the running direction (D2) thereof. The phase difference of the interference light at two locations can be detected with high resolution and precision, for instance, by photoelectrically converting the heterodyne interference light at the two locations with the detection system and comparing the phases of resulting two detection signals (optical beat signals).
When the object to be inspected comprises, for example, an optical projection system for an exposure apparatus, a portion of the light flux passed through the object to be inspected is employed as a reference light in the present invention so that, even if the exposing light to be employed for the exposure apparatus would have poor interference performance, e.g., like X-rays, the exposing light can also be employed as light flux for use upon inspection. This enables measurement including influences of coating layers in the object to be inspected. Further, the distribution of the phases of the wavefronts in the object to be inspected can be detected by a fine pitch by receiving the heterodyne interference light with a two-dimensional image pickup element of a CCD type and comparing the phases of the image pickup signals from each pixel.
More specifically, this feature of the inspection apparatus according to the present invention can provide advantages that the heterodyne interference light can be obtained so that the phase of the interference light between the light flux passed through the object to be inspected and the reference light generated from a portion of the light flux can be detected with high precision. Further, as the reference light (diffracting light) is generated from the light flux passed through the object to be inspected, light flux which is poor in interference performance can also be employed as an illuminating light for use upon detection. Therefore, particularly when an inspection is effected for the wavefront of an optical projection system with an interferometer in accordance with the present invention, which is employed with an exposing light such as ultraviolet rays or X-rays, the exposing light can also be employed as an illuminating light for inspection so that aberration can be evaluated with high precision under actually applicable conditions. Furthermore, a two-dimensional distribution of the phases of the interference light can be detected with high precision by taking a picture of an image of interference fringes with a two-dimensional image pickup element and detecting the phase of each pixel.
Furthermore, an alternative inspection apparatus for inspecting the optical device in accordance with the present invention may comprise a light path division system for generating two light fluxes having different light paths from the light flux passed through an object to be inspected, a pinhole-shaped diffracting member disposed on a light path of one of the two light fluxes (L0, L1) generated from the light path division system, a drive unit for continually transferring the light path division system, and a detection system for receiving a heterodyne interference light between the other light flux generated from the light path division system upon continually transferring the light path division system through the drive unit and the diffracting light generated with the diffracting element. This configuration can detect optical characteristics of the object to be inspected on the basis of detection signals from the detection system (as illustrated in FIGS. 3 and 4).
In the inspection apparatus according to the present invention, there may be employed, for example, a diffraction grating as the light path division system, which may be configured so as to implement frequency modulation by transferring the wavefront of the diffracting light generated from the diffraction grating upon continuously transferring the diffraction grating in the pitch direction. A velocity of varying the phase of the wavefront may vary with the degree of the diffracting light so that the interference light of two light fluxes to be generated from the diffracting light in different directions becomes a heterodyne diffracting light, thereby enabling the phase difference between two measurement points to be detected with high precision, as achieved with the inspection apparatus (as illustrated in FIGS. 1 and 2).
Moreover, an alternative inspection apparatus for inspecting the optical device in accordance with the present invention may comprise a diffraction grating of a predetermined pitch which can generate two light fluxes (L0, L1) having different diffraction degrees by diffracting the light flux passed through the object to be inspected, a pinhole-shaped diffracting member disposed on a light path of one of the two light fluxes generated from the diffraction grating, a drive unit for transferring the diffraction grating, and a detection system for receiving an interference light to be generated between the diffracting light generated from the diffracting element and one of the two light fluxes (L0, L1) generated from the diffraction grating upon stepwise transferring the diffraction grating with the drive unit at a predetermined number of times by the predermined pitech of 1/n (where n is an integer equal to or larger than three). This inspection apparatus is so adapted as to detect optical characteristics of the object to be inspected on the basis of detected signals of the detection system (as illustrated in FIG. 5).
The inspection apparatus according to the present invention can change the phase of the diffracting light by transferring the diffraction grating in the pitch direction as with the inspection apparatus according to the present invention (as illustrated in FIGS. 3 and 4), thereby varying the phase of the resulting interference light, too. This feature can detect the phase of the interference light with high precision by calculating the intensity of the interference light obtained upon changing the diffraction grating in plural positions. More specifically, the inspection apparatus according to the present invention can present the advantages that the phase of interference fringes at a desired measurement point can be detected with high precision by calculating the resulting detected signals because the phase of the interference light is stepwise transferred by a predetermined pitch amount. Furthermore, it can provide the merits that there can also be employed, as an illuminating light for inspection, a light flux which cannot otherwise be preferably employed for that purpose due to the fact that its interference performance is poor.
On the other hand, a method for manufacturing an exposure apparatus in accordance with the present invention may include a method for manufacturing an exposure apparatus for transcribing an image of a pattern formed on a mask onto a board through an optical projection system, which may be provided with an inspection adjustment step which includes a step for passing an exposing light through the optical projection system and a step for passing the exposing light sent from the optical projection system through a diffracting element, which generates diffracting light from a portion of the exposing light sent from the optical projection system during the movement of the diffracting element. The inspection adjustment step may further contain a step for detecting interference light between the diffracting light and an other exposing light sent from the optical projection system during the movement of the diffracting element.
In the inspection adjustment step of the method for manufacturing the exposure apparatus according to the present invention, the phase of the diffracting light as a reference light can be varied by the movement of the diffracting element, thereby perioridcally varying the phase difference between the diffracting light and another exposing light as a measuring light sent from the optical projection system. Therefore, a wavefront aberration of the exposing light sent from the optical projection system can be inspected and measured with high precision on the basis of interference fringes generated by interference between the diffracting light and the other exposing light.
Furthermore, the exposure apparatus according to the present invention may comprise an exposure apparatus for transcribing an image of a pattern formed on a mask onto a board through an optical projection system, which may be provided with an optical inspection device comprising a diffracting element for generating diffracting light from a portion of an exposing light sent from the optical projection system as an object to be inspected, which is disposed on a light path of the exposing light, a drive unit for driving the diffracting element, and an inspection unit for receiving interference light between the diffracting light and the other exposing light sent from the optical projection system during the movement of the diffracting element.